386 



THE SPECIAL SENSES. 



be obtained from the special works on anatomy or histology. Foi 

 the purposes of a physiological presentation the schematic figure 

 employed by Czermak and reproduced in Fig. 167 will suffice to 

 exhibit the general anatomical relations of the parts concerned in 

 the transmission of the sound waves from the exterior to the cochlea. 

 The Pinna or Auricle. The pinna opens into the external mea- 

 tus by means of a cone-shaped depression, the concha. The whole 

 organ, and especially the concha, may be considered as fulfilling 

 more or less perfectly the function of collecting the sound waves 

 and reflecting them into the meatus. In the lower animals the con- 

 cave shape of the ear and its motility probably make it much more 

 useful in this respect than in the case of the human ear. But even 

 in man the pinna is valuable to some extent in intensifying the 



appreciation of sounds and 

 also in enabling us to deter- 

 mine their direction. The ex- 

 ternal auditory meatus has a 

 length of about 21 to 26 mms., 

 and a capacity of something 

 over one cubic centimeter. 

 Its course is not straight, but 

 passes first somewhat back- 

 ward and upward, and then 

 turns forward and inward to 

 end against the tympanic 

 membrane. All sound waves 

 that affect the drum of the ear 

 must, of course, pass through 

 this canal. 



The Tympanic Mem- 

 brane. The tympanic mem- 

 brane closes the inner end of 

 the meatus and lies obliquely to the axis of the canal, its plane 

 making an angle, opening downward, of 150 degrees. The mem- 

 brane, although not more than 0.1 mm. thick, consists of three 

 coats : a layer of skin on the external surface, a layer of mucous 

 membrane on the side toward the middle ear, and in between a 

 layer of fibrous connective tissue. The middle layer gives to the 

 membrane its peculiar structure and properties. In form the 

 membrane has the shape of a shallow funnel with the apex, or 

 umbo > as it is called, somewhat below the center. The fibers of 

 the fibrous layer are arranged partly circularly and partly in lines 

 radiating from the umbo to the peripheral margin (Fig. 168). 

 The walls of the funnel are slightly convex outwardly; so that 

 each radiating fiber forms an arch. On the inner side of the mem- 

 brane the chain of ear ossicles is attached, so that the vibrations 



168. To show the structure of the 

 tympanic membrane, looked at from the side 

 of the meatus (Hensen) : ax, The axis of rota- 

 tion of the ear bones; d, the incus; a, the 

 head of the malleus. 



